1. Field of the Invention
The present invention relates to a novel organic charge transfer complex which has a high melting point, a solid electrolytic capacitor employing the same, and to a process for the preparation thereof. The present invention also pertains to a chip-type solid electrolytic capacitor employing said charge transfer complex.
2. Description of the Related Art
The development of digital devices in recent years has given rise to a demand for smaller size capacitors having a large-capacity which also have excellent high-frequency characteristics, and for chip-type capacitors which are indispensable in surface mounting on a printed wiring circuit.
Electrolytic capacitors mass produced by conventional methods and, which are known to have a large-capacity, include two types, the liquid electrolyte type in which a liquid electrolyte is impregnated, and the solid electrolyte type in which manganese dioxide is employed as a solid electrolyte. The former type of electrolytic capacitor utilizes ion conduction, and therefore resistance remarkably increases in the high frequency region with a corresponding disadvantageous increase in impedance, moreover this capacitor cannot be surface mounted by a soldering process. The latter type of electrolytic capacitor also has relatively high impedance in the high-frequency region partly because manganese dioxide has a relatively high specific resistance. Since this capacitor was subjected to thermal decomposition to obtain manganese dioxide from manganese nitrate, the oxide of film-forming aluminum electrode thereof was readily damaged, so that the leakage current of the capacitor remarkably increased.
In order to overcome the above-described disadvantages of conventional capacitors, one type of electrolytic capacitor which employs as a solid electrolyte a charge transfer complex consisting of a combination of 7,7,8,8-tetracyanoquinodimethane (hereinafter referred to as TCNQ) and a donor has already been proposed.
Examples of the donor employed in the proposed TCNQ charge transfer complex include N-n-hexyl-quinoline, N-ethylisoquinoline, N-n-butylisoquinoline (see Japanese Patent Public Disclosure No. 191414/1983), N-n-amylisoquinoline, N-isoamylisoquinoline (see Japanese Patent Public Disclosure No. 116552/1987), N-n-propylisoquinoline and N-isopropylisoquinoline (see Japanese Patent Public Disclosure No. 17609/1983).
However, the conventional TCNQ charge transfer complex which may be composed of the above-described compounds suffers from an inferior thermal stability and therefore melts or decomposes during the soldering process of surface mounting to become an insulator. Moreover a leakage current of a capacitor employing the conventional complex increases to a great extent during the soldering process because the oxide film is damaged. This prior art is also unsatisfactory in the manufacture of chip-type solid electrolytic capacitors.